1. Field of the Invention
The present invention relates to a ceramic heater which is used in an oxygen sensor and the like.
2. Description of the Related Art
Conventionally, in order to purify exhaust gas from vehicles or the like, oxygen sensors have been used to measure the concentration of oxygen in exhaust gas and adjust the ratio between air and fuel. Some kinds of oxygen sensors utilize an oxide semiconductor made of TiO.sub.2 the resistance of which changes depending on differences of, for example, oxygen concentration as the detecting device thereof, and other kinds of oxygen sensors utilize a solid electrolyte made of ZrO.sub.2 which generates electromotive force depending on differences of oxygen concentration as the detecting device. Because these sensors operate at high temperatures, it is necessary to raise the temperature of the detecting device. A ceramic heater including a metallic heating element made of Pt which is embedded in a ceramic sintered body such as Al.sub.2 O.sub.3 is used as a heating means. To use the sensors stably, it is necessary to restrict changes of the resistance of the heating element in the ceramic heater.
However, in ceramic heaters, cations such as Ca.sup.2+ or Mg.sup.2+ from CaO and MgO which are sintering assistants contained in the ceramic sintering body are transferred from the +electrode of the heating body to the -electrode thereof under high temperatures and high electric potential differences during use, and then deposited on the single electrode (migration). Due to the migration, the cations react with the heating body metal or the condition of the heating body changes, so that the resistance of the heating body increases thereby sometimes breaking wires or expanding the volume of the heating body to cause cracks in the ceramic sintered body. Consequently, the durability of the ceramic heater is deteriorated. For this reason, the changes of the resistance of the heating body due to migration are a significant problem concerning the stable use of sensors.
Japanese Patent Publication No. 3-183942 has disclosed a ceramic heater in which the heating body is formed in a parallel pattern. The intention concerned with the ceramic heater thereof is to decrease an electric potential difference applied inbetween the patterns thereby preventing migration by forming the heating body in the parallel pattern.
Japanese Patent Publication No. 4-249089 has disclosed a ceramic heater which utilize a ceramic sintered body of 98% or more by weight of alumina and 2% or less by weight of a sintering assistant. The purpose of the ceramic sheet of this invention is to decrease the absolute quantity of cations such as Ca.sup.2+ under high temperatures and high electric potential difference in order to prevent migration.
However, if it is intended to form the heating body of the parallel pattern as described above, the design process of such a pattern for obtaining excellent soaking characteristics becomes complicated and further, disparity of production quality increases as compared with conventional series patterns, making it very difficult to produce the heating body.
If the sintering assistant employs a ceramic sintered body of 2% by weight or less, it becomes difficult to sinter the ceramic sintered body, so that it becomes necessary to pulverize the ceramic powder and increase the sintering temperature thereby leading to increased production costs.
Although it is possible to devise a method for decreasing the electric potential difference applied to the heating body by reducing the resistance of the heating body by increasing the film thickness of the heating body, this method induces the increase of production costs when a precious metal such as Pt is used as the heating body.